There are various inkjet recording methods such as a method of using electrostatic attraction to eject ink (electric-field control method), a method of using pressure generated by driving a piezoelectric element to eject ink (drop-on-demand method or pressure pulse method), a method of using pressure generated by generating and growing bubbles at high temperature (bubble or thermal jet method), and the like. These methods are known to provide a highly accurate image.
The types of ink typically used in these inkjet recording methods include a water-based ink produced by use of water as a prime solvent and an oil-based ink produced by use of oil as a prime solvent. The image printed by use of the water-based ink is of inferior waterproof properties and the water-based ink is not suitable for printing on a record medium having a waterproof surface. In contrast, the oil-based ink has advantages such as that of providing a printed image with high waterproof properties and of facilitating the printing on a record medium having a waterproof surface and on a sheet of woodfree paper. In particular, an oil-based ink including pigment as a colorant also has high lightfastness properties.
However, the conventional oil-based ink includes, typically, as an organic solvent, aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as hexane and coal oil, ketones such as methyl ethyl ketone, esters such as acetic ester, or low-boiling solvent such as propylene glycol monomethyl ether acetate. The printing process using ink including such an organic solvent involves problems with easy clogging of a nozzle because the organic solvent has a low boiling point and a low flashpoint and dries at excessively high speeds.
In addition, there is another problem with an increase of the cost required for meeting printer specifications for the reasons of solubility and the bloating tendency with respect to the plastic (e.g., a polystyrene resin, an ABS resin or the like) used for an ink container and units or components in a printer or the like. Further, printing on a polyvinyl chloride base material is dissatisfactory in the quality of printing and the drying properties after printing.
It can be said that a solvent of glycol ether dialkyl ethers is superior to a low boiling-point solvent generally used as a solvent for ink compositions, since the solvent of glycol ether dialkyl ethers has high safety because of a high boiling point, a low vapor pressure and a low odor and is outstanding at the working atmosphere. However, the solvent of glycol ether dialkyl ethers has a lower resin solubility as compared with the conventionally used solvent with a low boiling point. For this reason, when a commercially available solid resin is melted and used, the number of usable resins is limited. As a result, the use of a resin having a high glass transition temperature (Tg) such as methyl methacrylate homopolymer is difficult. The use of such an acrylic resin involves the problems in the stabilities of the storage and ejection of ink.
Alternatively, acrylic monomers having the effect of increasing the solubility are used to cause a reduction in Tg or a reduction in molecular weight in order to produce an acrylic resin with an improved solubility. The use of this acrylic resin involves the difficulties in sufficiently providing the drying properties after printing and the rubfastness.